A hot-rolled steel strip which has passed through a finishing rolling for a hot-rolling process (hereinafter, referred to as “steel strip”) is transported from a finishing rolling mill to a coiler by using a run-out table. The steel strip under the transportation is cooled to a predetermined temperature by means of cooling devices which are provided above and under the run-out table, and then, is coiled by the coiler. Since the cooling manner of the steel strip after passing through the finishing rolling has a significant influence on the mechanical property of the steel strip, it is important to uniformly cool the steel strip to a predetermined temperature.
Usually, the cooling of the steel strip after passing through the finishing rolling is carried out by using, for example, water (hereinafter, referred to as “cooling water”) as a cooling medium. In this case where the steel strip is cooled with the cooling water, a cooling state of the steel strip changes depending on the temperature of the steel strip. For example, in a general laminar cooling process, as illustrated in FIG. 9, (1) when the surface temperature T of the steel strip is not lower than approximately 600° C., the steel strip is cooled in a film boiling state A, (2) when the surface temperature T of the steel strip is not higher than approximately 350° C., the steel strip is cooled in a nucleate boiling state B, and (3) when the surface temperature T of the steel strip is in the temperature range between the film boiling state A and the nucleate boiling state B, the steel strip is cooled in a transition boiling state C. Here, the “surface temperature” means the temperature of a steel strip surface being cooled with the cooling water.
In the film boiling state A, when the cooling water is ejected onto the steel strip, the cooling water immediately vaporizes on the surface of the steel strip, whereby a vapor film covers the surface of the steel strip. When the steel strip is cooled in the film boiling state A, since this vapor film cools the steel strip, a cooling performance is low but the coefficient of heat transfer h is substantially constant, as illustrated in FIG. 9. Therefore, as illustrated in FIG. 10, the heat flux (heat flow rate) Q decreases as the surface temperature T of the steel strip decreases. Generally, in a case where the inside temperature of the steel strip is high, the surface temperature is also high due to the heat conduction from the inside of the steel strip. Accordingly, in the film boiling state A, a portion of the steel strip where the surface temperature is high rapidly cools down, and a portion of the steel strip where the surface temperature is low slowly cools down. As a result, even if the inside temperature or the surface temperature of the steel strip is locally varied, the temperature deviation in the steel strip decreases as the cooling proceeds.
In the nucleate boiling state B, when the cooling water is ejected onto the steel strip, the cooling water comes into direct contact with the surface of the steel strip without generating the above-described vapor film. Therefore, the coefficient of heat transfer h of the steel strip cooled in the nucleate boiling state B is higher than the coefficient of heat transfer h of the steel strip cooled in the film boiling state A, as illustrated in FIG. 9. In addition, as illustrated in FIG. 10, the heat flux Q decreases as the surface temperature of the steel strip decreases. Accordingly, in the nucleate boiling state B, the temperature deviation in the steel strip decreases as the cooling proceeds, as in the film boiling state A. Meanwhile, the heat flux Q (W/m2) can be calculated by using the following Formula (1), where the h (W/(m2·K)) is the coefficient of heat transfer, the T (K) is the surface temperature of the steel strip, and the W (K) is the temperature of the cooling water ejected onto the steel strip.Q=h×(T−W)  Formula (1)
However, in the transition boiling state C in which a film boiling state portion and a nucleate boiling state portion are generated, a portion cooled through the vapor film and a portion brought into direct contact with the cooling water coexists. In this transition boiling state C, the coefficient of heat transfer h and the heat flux Q increase as the surface temperature of the steel strip decreases. This is because the contact area between the cooling water and the steel strip increases as the surface temperature of the steel strip decreases.
Accordingly, a portion where the surface temperature T of the steel strip is high, that is, a portion where the inside temperature is high slowly cools down, while a portion where the surface temperature T of the steel strip is low rapidly cools down. As a result, if a local temperature variation occurs in the steel strip, this temperature variation significantly increases. That is, during the cooling of the steel strip in the transition boiling state C, the temperature deviation in the steel strip increases as the cooling proceeds, thus, it is impossible to achieve the uniform cooling of the steel strip.
Patent Document 1 discloses a method including a step that stops cooling before reaching a transition boiling start temperature, and a step that subsequently cools the steel strip with cooling water in the water amount density (amount of water per unit area and unit time supplied on the steel strip) by which the cooling water becomes the nucleate boiling state. In this cooling method, based on the fact that the transition boiling start temperature and the nucleate boiling start temperature shift to the higher temperature side as the water amount density of the cooling water ejected onto the steel strip increases, after cooling the steel strip in the film boiling state, the steel strip is subsequently cooled in the nucleate boiling state by increasing the water amount density of the cooling water.